Vehicles, having an electric drive mechanism, are particularly used when an emission must be prevented. This is specially desired in industrial sheds and in towns or cities not only in case of smog danger. In many towns, whole areas are allowed only for motor vehicles, having electric drive mechanism, particularly city autobuses being provided with such electric drive mechanisms.
As energy source for the electric drive mechanism are usually considered lead storage batteries to which are attached on electric motors which drive the wheels of the motor vehicle. Direct current shunt motors of easily controllable rotational speed are adequate for conventional vehicles with electric drive mechanism.
In purely electric vehicles such as city autobuses which have easily replaceable batteries or rechargeable batteries, a real disadvantage is the small range, the heavy weight and the large volume of the storage batteries. For this reason, hybrid vehicles have been proposed which have, on one hand, an electric drive mechanism with electric motor and chargeable batteries and, on the other, an internal combustion engine for driving a generator which charges the battery of the vehicle. Compared to conventional pure electric drive mechanisms, hybrid drive mechanisms have the advantage that problems of limited range and of long periods of time for recharging the batteries can be overcome; but a hybrid drive mechanism does not remove the problem of volume and weight, but rather increases them by the additional internal combustion engine.
In DE A 44 44 545 has been disclosed a hybrid vehicle, wherein the hybrid vehicle has an internal combustion engine for driving the vehicle, an electric motor for driving the vehicle, a battery for feeding the electric motor and a generator different from the electric motor for charging the battery. This is a so-called parallel hybrid vehicle where the battery is charged via the electric motor which, while the vehicle is driven, can be shifted by the internal combustion engine as generator. Therefore, such a parallel hybrid vehicle is a vehicle which can optionally be used for short distances (up to about 100 km) as pure electric vehicle, but otherwise is designed as vehicle with internal combustion engine and also predominantly used therewith.
DE A 195 23 985 discloses a control device for such a hybrid vehicle having as source of energy one electric motor and one internal combustion engine, wherein energy produced by the prime mover is applied to a motor/generator, via a generator inverter, so that the motor/generator drives the internal combustion engine in order to apply thereto a braking force. In this manner, energy which has been produced during the regenerative braking and cannot be absorbed by a saturated batter, e.g. when the vehicle moves downhill, is used in order to assist the braking of the vehicle. At the time, when the vehicle moves on longer steep inclination, the regenerative braking of the motor is used to drive the vehicle and, as a result, the charging of the battery is assisted by the current produced by the regenerative braking, the battery easily reaching its saturated state. Accordingly, even when the regenerative operation of the motor is carried out, it is possible, after the battery has reached the charge saturation state, no more to charge it; accordingly, its current level is reduced and the braking force of the motor thereby is less.
One other problem in motor vehicles, having an electric drive mechanism, is that the electric drive mechanism must not exceed a maximum admissible rotational speed, since the mechanical strength is ensured only up to that point. But in electric vehicles, the rotational speed of the drive mechanism over the road is coupled with the speed of the vehicle. However, under certain driving conditions, such as in a downhill drive of longer duration, there is the danger that said admissible rotational speed be exceeded even though no drive torque actuates the wheels, especially in vehicles having great weight and low maximum speed as is the case in city autobuses. Due to the heavy weight, corresponding slope output forces appear while as result of the relatively low maximum speed a correspondingly lower tractional resistance simultaneously counteracts the weight so that rotational speed above the maximum rotational speed limit admissible can occur.